Had hesitation under increasing throttle. Cat is 180,000 miles old. Getting a PO420 code. Changed wires. Inspected plugs and re-gaped. Plugs were very clean and dry. Could a misfire cause a PO420 code, or a poorly performing Cat cause the hesitation (misfire)?

 

Are you saying you have hesitation due to a known misfire under accelleration, is this mostly from a stop or also accel at highway speeds. Due you have an EGR valve on this motor? If so, does it have a vacuum hose going to it or any of the EGR System components, if so, unplug the hose, cap it off and re-road test.

A misfire could make the Cat stop working properly causing the P0420 Cat Efficiency code, fix the misfire first, the misfire could be ignition related, lean fuel related or EGR related.

Ignition/timing related will show up most from a dead stop accelleration or a steady highway cruise then accellerate, a lean fuel issue should show up better at really high speed but dont go out and get into an accident, more to come on lean operation after you let us know about the misfire scenario and EGR diagnosis.